The present invention relates to a method for producing a phosphoric monoester by phosphorylating an organic hydroxy compound (hereinafter referred to as ROH). Particularly, the present invention is concerned with a method for producing phosphoric esters containing a monoester in a high concentration, which comprises a first step of esterification of ROH which esterification is conducted in the presence of an excess amount of a phosphoric acid component, and a second step of reaction which is conducted by adding a further portion of ROH to the reaction mixture of the first step to make the overall ROH equivalent to its stoichiometric quantity.
Acidic phosphoric esters of ROH and their alkali metal salts, ammonium salts, alkanolamine salts are widely used as cleansing agents, emulsifying agents, antistatic agents, rust inhibiters, etc.
Acidic phosphoric esters presently industrially widely used are each the equimolar mixture of a monoester(I) and a diester(II) (hereinafter referred to as a sesquiphosphate) prepared from ROH and phosphorus pentoxide. ##STR1## However, the monoester and the diester are quite different in their physical properties. For example, with respect to alkali metal salts and alkanolamine salts of a long chain alkylphosphate, the monoester thereof has good water solubility, good foaming property, good cleansing power and low toxicity and gives little irritation to the skin, and accordingly it provides an excellent cleansing agent, whereas the diester thereof is hardly soluble in water, exhibits little foaming power or rather has a defoaming property, and accordingly it can not be used as a high foaming cleansing agent. Accordingly, the sesquiphosphate does not provide the above properties of the monoester, and therefore can not be a substitute for the monoester where the properties of the monoester are required.
Thus, it is strongly desired to produce safely, easily and industrially a phosphoric ester having a high monoester content. There are some reports in this respect, as follows:
(1) A method which comprises hydrolysing a monoalkyl phosphorodichloride ##STR2## prepared by reacting ROH with phosphorus chloride. (Fuben Beil: Methoden der organischen Chemie, Vol. 12/2, p. 163-164, Japanese laid-open patent application No. 64226/75) PA0 (2) A method which comprises adding water to ROH preliminarily and reacting phosphorus pentoxide therewith. (Japanese Patent Publication No. 14416/66) PA0 (3) A method which comprises reacting an alcohol with orthophosphoric acid and phosphorus pentoxide separately or simultaneously. (Japanese Patent Publication No. 6730/67) PA0 (4) A method which comprises reacting ROH with polyphosphoric acid [A. K. Nalson et al.: Inorganic Chemistry, Vol 2, 775-777 (1963), F. B. Clarke et al.: J. Am, Chem, Soc. 88, 4401-4405 (1966)]
However, the above mentioned methods have the following drawbacks and accordingly they are industrially disadvantageous.
The method identified by (1) produces 3 moles of hydrochloric acid to obtain 1 mole of the monoester, as shown by the following formulas, and accordingly there are difficult problems such as corrosion of the apparatus and disposal of the hydrochloric acid. ##STR3## Further, the method (1) generates an alkyl chloride as a by-product, which adversely affects the yield of the monoester.
With respect to the methods (2) and (3) and referring to the proportion of the monoester and the diester, it is possible to increase the proportion of the monoester by increasing the amounts of water or orthophosphoric acid. However, then the yield of inorganic orthophosphoric acid will be extremely increased. Namely, the reaction rate of phosphoric acid is extremely low, and the product contains a great amount of inorganic orthophosphoric acid, which is undesirable in certain cases and thus limits the field of usage of the product.
In the method (4), the yield of inorganic orthophosphoric acid produced as a by-product of the reaction, almost corresponds to the reciprocal number of the average condensation degree of the polyphosphoric acid, and accordingly to order to minimize the yield of the inorganic orthophosphoric acid, it is necessary to use polyphosphoric acid having a very high condensation degree. However, it is extremely difficult to industrially produce such a high condensation polyphosphoric acid in view of the limitation in the materials for the vessel for its production.